1. Field of the Invention
The present invention relates generally to an RF switch, and in particular, to an RF switch suitable as a switch for switching between a transmission signal and a received signal at a signal Tx/Rx end in a Time Division Duplexing (TDD) system.
2. Description of the Related Art
2nd and 3rd generation mobile communication systems are mostly based on Frequency Division Duplexing (FDD). A duplexer separates a Tx signal from an Rx signal in FDD. In comparison, 3.5th and 4th generation mobile communication systems are expected to use TDD.
In time-division transmission schemes including TDD, communications are carried out with the same radio frequency in time division between transmission and reception. One frame is divided for transmission and reception, for bi-directional communications at one frequency.
FIG. 1 is a block diagram of Tx and Rx ends in a typical TDD system. Referring to FIG. 1, a Tx signal is amplified to an appropriate power level in a power amplifier 40 and then radiated through an antenna (ANT) 60 through a Tx-Rx switch 10 and a full-band filter 50. For reception, a signal received through the ANT 60 passes through the full-band filter 50 and the Tx-Rx switch 10 and then is amplified to an appropriate power level in a Rx power amplifier (e.g. LNA) 20. The Tx-Rx switch 10 switches between transmission and reception according to a switching control signal received from a controller (not shown).
As described above, transmission alternates with reception on the same frequency every predetermined time period. That's why a high-speed Tx-Rx RF switch is needed.
Due to the requirement of high-speed switching, the RF switch uses a switch based on a semiconductor device such as a PIN diode or a Field Effect Transistor (FET) rather than a mechanical switch. However, the semiconductor switch is not feasible for high power because of the vulnerability of semiconductor to high power.
When a high power is applied, much heat is created and thus without sufficient heat proofness, the switch is destroyed. An RF switch developed to withstand high power is equipped with a separately procured cooler and thus the RF switch is very expensive and hard to fabricate. Therefore, the RF switch is limited to military use.
To solve this problem, the TDD system typically adopts a circulator instead of an RF switch to separate a Tx signal from an Rx signal. However, the use of the circulator also has the distinctive drawback that sufficient isolation from a Tx signal during reception is difficult to achieve and in an open state caused by some defects in an antenna during transmission, the Tx signal is introduced into a receiver, thereby causing system malfunction or seriously decreasing the quality of the Rx signal. Also, Tx Passive Intermodulation Distortion (PIMD) is caused, thereby adversely affecting propagation quality from other service providers.